Electrophoretic display device

ABSTRACT

An electorphoretic display and/or recording device in which an electrophoretic suspension layer including a dispersion of at least one electrophoretic material in a finely divided powder form suspended in a suspending medium is interposed between a pair of electrodes. An electric field is imposed across the electrophoretic suspension layer to change the optical reflective property of the suspension layer by changing the spatial distribution of the electrophoretic material in the suspending medium electrophoretically.

O United States Patent 1151 3,668,106 Ota 1 June 6, 1972 [541ELECTROPHORETIC DISPLAY DEVICE 3,384,565 5/1968 11113 111 et a1...204/l8l 3,477,934 1 1/1969 Carreira et al..... ..204/300 X [m3,510,419 5/1970 Carreira et al ..2o4/1a1 [73] Assignee: MltallhltaElectric Industrial Co., Ltd 3,511,651 5/1970 Rosenberg ..204/ 180 XOsaka, Japan z A 9 1970 Primary Examiner-John H. Mack [22] ed FAssistant Examiner-A. C. Prescott [21] Appl- 26,374 Attorney-Wenderoth,Lind 8: Ponack 521 US. c1. ..204/299, 204/180 R, 204/181 [571 ABSTRACT[5|] 5/02C23b 13/00 An electorphoretic display and/or recording devicein which [58] "204/ 299; 96/] A an electrophoretic suspension layerincluding a dispersion of at least one electrophoretic material in afinely divided [56] Rm powder fonn suspended in a suspending medium isinterposed UNITED STATES PATENTS between a pair of electrodes. Anelectric field is imposed across the electrophoretic suspension layer tochange the opt1- 3,102,026 8/1963 Metcalfe et a1. ..96/1 X calrgflgcfivg properly of the suspension layer by changing the 3,145,1561964 204/299 X spatial distribution of the electrophoretic material inthe 312441083 4/1966 Gllndladlu x suspending medium electrophoretically.3,268,331 8/1966 Harper .96/] 3,271,145 9/1966 Robinson ....96/1

25 Claims, 23 Drawing Figures PATENTEDJun 6 m2 SHEET 3 BF 5 FIGLSQ FIGSbINVENTOR ISAO OTA BY Mw%,/im/5m4 ATTORNEYS PATENTEDJUH 6 1912 3,668,106

SHEET u 0F 5 FIGIO INVENTOR I SAO OTA ATTORNEYS ELECTROPHORETIC DISPLAYDEVICE BACKGROUND OF THE INVENTION This invention relates to a displayand/or recording device and particularly to an electrophoretic displayand/or recording device comprising at least one electrophoretic materialsuspended in a suspending medium.

A cathode ray tube is now widely used as an electric display apparatusbecause of its high speed, ease with which it can be scanned, and so on,but it still has come disadvantages. For example, a cathode ray tube isdifiicult to produce so that it has a flat panel or so that it has alarge size. In addition, it requires a high voltage to operate.

There have been proposed many electric panel display apparatuses such asan electroluminescent panel, arrays of luminescent diodes, incandescentlamps or plasma cells. They are all self-luminescent types, but are notin widespread use because of inferiority in, for example, luminescentefficiency, life or cost. Further, it is difficult for them to memorizeor record the displayed image. Liquid crystals are a possible medium ofwhich to constitute a flat panel display apparatus capable of changingcolor by a change in an electric field or change in heat. At presentthey are not actually used because their properties are not entirelysatisfactory for display purposes.

The electrophoresis phenomenon is widely used in electrodeposition offine particles or development of electrostatic latent images inelectrophotography. The prior art, for example U.S. Pat. Nos. 3, 145,156to Oster, No. 2,940,847 to Kaprelian, and No. 3,384,488 to Tulagin andCarreira describe methods of producing images by utilizingelectrophoresis or photoelectrophoresis.

According to these prior art methods, charged particles in thesuspending medium are transported to the surface of an electrode or asheet so as to reproduce a pattern corresponding to that of an inputelectric field or to an input light image. The visible image can beobtained by removing the electrode or a sheet.

The prior art is not aimed at a variation in the optical reflectiveproperty of a suspension itself with a change in the spatialdistribution of electrophoretic particles in the suspension. In otherwords, the prior art relates essentially to the reproduction ofpermanent visible images but not to a changable display system.

SUMMARY OF THE INVENTION 7 An object of this invention is to provide adevice for display and/or recording, which comprises an electrophoreticsuspension layer.

A further object of this invention is to provide a large and/or flatdisplay panel comprising an electrophoretic suspension layer A stillfurther object of this invention is to provide a flexible panel fordisplay and/or recording, which comprises an electrophoretic suspensionlayer.

These objects are achieved by a device for display and/or recordingaccording to the present invention which comprises a suspension layerincluding a dispersion of at least one electrophoretic material in afinely divided powder form suspended in a suspending medium, saidsuspension layer having two opposite major surfaces; a first electrodeand a second electrode which are opposed to each other and are coupledto said two opposite major surfaces of said suspension across saidsuspension layer through said electrodes, said electric field changingthe spatial distribution of said electrophoretic material of saiddispersion to change the optical reflective property of said suspensionlayer.

Other objects and advantages of this invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

In the drawings:

FIGS. la and lb are cross-sectional views of a display and/or recordingpanel in accordance with this invention;

FIGS. 2a and 2b are cross-sectional views of another embodiment of thisinvention;

FIGS. 3a and 3b are cross sectional views of still another embodiment ofthis invention;

FIGS. 4a and 4b are cross sectional views of another embodiment of thisinvention;

FIG. 5 is a schematic perspective view, partially broken away, of apattern display panel according to this invention;

FIG. 6a is a front view of a first electrode for use in a characterdisplay panel according to this invention;

FIG. 6b is a cross sectional view of the panel of FIG. 6a;

FIG. 6c is a front view of another first electrode for use in acharacter display panel according to this invention;

FIG. 7 is a schematic perspective view, partially broken away, of animage display panel according to this invention;

FIGS. 8a, 8b and 8c are schematic cross sectional views of anelectrostatic image display and/or recording panel according to thisinvention;

FIG. 9 is a cross sectional view of an image display device according tothis invention;

FIG. 10 is a schematic cross-sectional view of an electrostatic imagedisplay and/or recording panel according to this invention;

FIG. 1 1 is a cross sectional view of a display panel according to thisinvention;

FIGS. 12a and 12b are cross sectional views of a display panel accordingto this invention;

FIG. is a schematic front view of a display panel according to thisinvention; and

FIG. 13 is a schematic perspective view of a sheet with holes for use inthe panel of FIG. 12.

The size and shapes of elements of the drawings should not be consideredas actual sizes or even proportional to actual sizes because manyelements have been purposely distorted in size or shape in order to morefully and clearly describe the invention.

Referring to FIG. la, reference character 1 designates, as a whole, adisplay and/or recording panel which has an electrophoretic suspensionlayer 2 therein.

When said suspension layer 2 is in a fluid state, it is enclosed in ahousing 3 having a frame 38 and two opposed major housing walls 4 and 5which are transparent.

Said suspension layer 2 has two opposite major surfaces along said twoopposite major housing walls 4 and 5 and includes a dispersion of atleast one electrophoretic material 6 in a finely divided powder formsuspended in a suspending medium 7. The particles of the material 6 areshown greatly enlarged in this and subsequent figures for clarity ofillustration. Said two opposite major surfaces of said suspension layer2 are in contact, respectively, with a first electrode 8 and a secondelectrode 9 which are transparent and are attached to the inner surfacesof said two opposite major housing walls 4 and 5.

Said first electrode 8 and second electrode 9 are connected to outputterminals of a DC. voltage applying means 10 through apolarity-reversing-switching-device 11. Before an electric field issupplied to said suspension layer 2 from said DC. voltage applying means10, the electrophoretic material 6 in a finely divided powder form isdistributed uniformly throughout the suspending medium 7 as shown inFIG. 1a. When the electrophoretic material 6 is, for example, white, andthe suspending medium 7 is, for example, black, the suspension layer 2appears gray under the illumination of incandescent lamp. When the graysuspension layer 2 is subjected to a DC. electric field by said DC.voltage applying means 10, the electrophoretic material 6 is caused tomove electrophoretically in a direction either to a cathode or to ananode depending upon its polarity. For example, if the material 6 has anegative polarity, it moves and is deposited on the anode 8 and has aspatial distribution different from the uniform distribution as shown inFIG. 1b. 8

The resultant suspension layer 2 has a different spatial distribution ofmaterial 6 and a different optical reflective property from the originalsuspension layer 2 having the uniform distribution of the material 6.The display and/or recording panel 1 is thus white on the anode side andblack on the cathode side.

The color characteristic of the display and/or recording panel 1 can bereversed by reversing the polarities of the applied voltage. When only asmall amount of the electrophoretic material is deposited on the anode,which amount is insufficient to hide the resultant suspension, the panelhas on the anode side, a halftone appearance dependent upon the hidingpower of the electrophoretic material deposited on the anode. By hidingpower of the suspension layer is meant the degree to which thesuspension layer deposited on a base substance hides it. Therefore, thecolor characteristic at the anode side can be controlled continuously bycontrolling the amount of the electrophoretic material deposited on theelectrode. The amount of the electrophoretic material deposited on theelectrode can be controlled by the strength or the length of time ofapplication of a DC. voltage. The color characteristic at the cathodeside is also changed by the electrophoretic movement of theelectrophoretic material to the anode. In such a way, the display and/orrecording panel 1 of the present invention can have its colorcharacteristic changed by varying the strength, the length of time orthe polarity of an applied electric field.

Electrophoretic material deposited on an electrode surface byelectrophoresis will stay on the electrode even after removal of anapplied electric field. This means that a display device according tothe present invention employing electrophoresis of the electrophoreticmaterial suspended in a suspending medium can memorize outputinformation without using further electric power. The panel can have theoriginal color characteristic restored by applying an appropriate D.C.electric field having a reverse polarity to that of the initial D.C.electric field or by applying strong mechanical vibrations to thedevice. Application of an AC. electric field to the suspension layer caneffectively restore the original color characteristic of the panel.

The suspending medium 7 can be prepared so as to have the desired colorby dissolving a colored substance, such as a dye, in a colorless liquidor by suspending electrically neutral colored particles, such as dyes orpigments in a colorless liquid. For example, deep blue coloredsuspending medium can be prepared by dissolving oil black dyes in ethyleacetate or kerosene. Violet, brown or green colored suspending mediumcan be prepared by dissolving cobalt naphthenate, manganese naphthenateor nickel naphthenate in trichlorotrifluoroethane, respectively.

Referring to FIG. 2a wherein similar reference numbers designatecomponents similar to those of FIG. 1, a colored porous layer 12 isinserted in a colorless suspending medium 13. In the device of FIG. 2a,a color which is a mixture of the colors of the electrophoretic material6 and the colored porous layer 12 can be seen from both electrodes.

When a DC. electric field is applied across the two electrodes 8 and 9,the electrophoretic material 6 is caused to pass through the coloredporous layer 12 and to be deposited on one electrode, for example theanode, depending upon its polarity, as shown in FIG. 2b. When a layer ofthe electrophoretic material 6 deposited on the anode has enough hidingpower, the device of FIG. 2b has the same color as the electrophoreticmaterial 6 on the anode side. When the colored porous layer 12 can alsohide a layer of the electrophoretic material 6 deposited on the anode,the device of FIG. 2b has almost the same color as the colored porouslayer 12 on the cathode side.

The color characteristic can be reversed by reversing the polarity ofthe applied potential. It is also possible to use an electrophoreticsuspension layer having a colored porous layer inserted in a coloredsuspending medium and at least one electrophoretic material suspendedtherein. Colored porous layer 12 in the present invention can be madefrom any sheet material having pores therein. The size of the pores mustbe large enough to pass the particles of electrophoretic materialtherethrough and small enough to hide the electrophoretic material fromsight. Operable materials are a cloth or a mesh woven of natural orartificial fibers; a fibroid sheet having thousands of irregular pores;a thin plate with a lot of tiny holes; and a sheet having granularmaterial bound together with resin or an adhesive agent to form a largenumber of pores.

Referring to FIGS. 3a, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a suspendingmedium 15 which is colored or colorless includes at least two kinds ofelectrophoretic materials 16 and 17 in a finely divided powder form. Thesuspending medium 15 and the materials 16 and 17 together make up theelectrophoretic suspension layer 18. Said two kinds of electrophoreticmaterials 16 and 17 are different from each other with respect to thecharge polarity and the optical reflective property.

The device of FIG. 3a displays at the opposite sides a color which is amixture of the colors of said two kinds of electrophoretic materials 16and 17, and the color of the suspending medium 15.

When a DC. electric field is applied to the electrophoretic suspensionlayer 18, said two kinds of electrophoretic materials 16 and 17 arecaused to move electrophoretically in directions opposite to each other.One material, that with a positive polarity, moves toward the cathodeand is deposited on the cathode, and the other, that with a negativepolarity, moves to and is deposited on the anode, as shown in FIG. 3b.When one of said electrophoretic materials 16 with positive polarity is,for example, yellow, and the other of said electrophoretic materials 17with negative polarity is, for example, cyan, the device produces aspatial distribution of electrophoretic materials 16 and 17 as shown inFIG. 3b and it is colored yellow at the cathode side and cyan at theanode side.

Before the device is subjected to the DC. electric field, the device hasa green color, if the suspending medium 15 is colorless, at bothelectrodes due to a uniform spatial distribution of the yellow material16 and the cyan material 17, as shown in FIG. 3a.

The color characteristic of the display and/or recording device can bereversed by reversing the polarity of the applied voltage.

Referring to FIG. 4a wherein similar reference numbers designatecomponents similar to those of the foregoing figures, an electrophoreticsuspension layer 21 includes a suspending medium 15 which is colored orcolorless and at least two kinds of electrophoretic materials 19 and 20in a finely divided powder form. Said two kinds of electrophoreticmaterials 19 and 20 have the same charge polarity but have differentelectrophoretic mobilities and optical reflective properties from eachother. The device of FIG. 4a initially has, at both sides, a color whichis a mixture of the colors of said two kinds of electrophoreticmaterials 19 and 20 and the color of the suspending medium 15. When saidtwo kinds of elec' trophoretic materials 19 and 20 are, for example,white and black, respectively, and the suspending medium 15 iscolorless, the device is colored gray at both sides. When a DC. electricfield is applied to the suspension layer 21, said two kinds ofelectrophoretic materials 19 and 20 are caused to moveelectrophoretically in the same direction.

When electrophoretic materials 19 and 20 are positively charged and theelectrophoretic mobility of said electrophoretic material 19 is greaterthan that of said electrophoretic material 20, the electrophoreticmaterial 19 moves faster in the suspension layer 21 under the effect ofa DC. electric field than does the electrophoretic material 20. Theamount of the former deposited nearest to the cathode is larger than theamount of the latter, as shown in FIG. 4b. Therefore, the device has adeeper white color at the cathode side and a deeper black color at theanode side than does the original device to which no D.C. electric fieldis applied.

The color of the display and/or recording device can be reversed byreversing the polarity of the applied voltage. As is apparent from theforegoing description and figures, the color of the suspension layeraccording to this invention can be changed. This is because the spatialdistribution of electrophoretic material in the suspension is changed byapplication of an electric field and a layer of at least oneelectrophoretic material deposited on an electrode hides or is hidden bythe residual colored component in the suspension, such as the coloredsuspending medium, a colored porous layer or another electrophoreticmaterial.

In the devices of the foregoing figures, it is not always necessary thatboth of the aforesaid two opposite major housing walls and both of theaforesaid first electrode and second electrode be transparent. It ispossible to prepare a display and/or recording device which shows achange in color only at one side by employing one transparent housingwall and one transparent electrode corresponding to said one transparenthousing wall.

Referring to FIG. 5, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a suspension layer22 can be any possible electrophoretic suspension layer, such as thesuspension layer 2, 14, 18 or 21 of the FIGS. 1a, 2a, 3a or 4a. Thesuspension layer 22 includes at least one electrophoretic materialsuspended in a suspending medium and is enclosed in a housing 3 havingtwo opposite major housing walls 4 and 5, at least one of which istransparent. In the drawing, housing wall 4 is transparent. The firstelectrode 23 has a pattern, such as an E- shaped pattern. The secondelectrode 24 is transparent and extends uniformly across the whole ofthe transparent housing wall 4.

When a D.C. electric field from a source is applied across said firstelectrode 23 and said second electrode 24 through switch 11 in a mannersimilar to that described above, the E shaped pattern is displayed onsaid transparent electrode 24 due to electrophoretic movement ofelectrophoretic material. A change in the polarity of the applied D.C.electric field results in a change in the color of said E shapedpattern.

Referring to FIGS. 6a and 6b, wherein similar reference numbersdesignate components similar to those of the foregoing figures, anelectrophoretic suspension layer 22 can be any possible electrophoreticsuspension layer, such as one of the suspension layers 2, l4, 18 or 21of FIG. la, 2a, 3a or 4a. The suspension layer 22 includes at least oneelectrophoretic material suspended in a suspending medium and isenclosed in a housing 3 having two opposite major housing walls 4 and 5.

A first electrode 25 is composed of a plurality of segmental electrodesS 5 which are separated from each other. A second electrode 24 istransparent and extends uniformly across the whole of the transparenthousing wall 4. Said plurality of segmental electrodes S, 5 areconnected through leads to electrical terminals T T positioned at theouter surface of said housing wall 5 as shown in FIG. 6b. Differentcombinations of said plurality of segmental electrodes S S-, willdisplay desired numerical characters when a D.C. electric field isapplied across the selected segmental electrodes of said first electrodeand the second electrode 24. For example a D.C. electric field appliedacross the second electrode 24 and the segmental electrodes S S S S andS of the first electrode will cause the device to display the numericalcharacter 3.

There is another method of connecting the plurality of segmentalelectrodes S S to the tenninals. Referring to FIG. 60, segmentalelectrodes S S of the first electrode will cause the device to displaythe numerical character 3.

There is another method of connecting the plurality of segmentalelectrodes S S to the terminals. Referring to FIG. 6c, segmentalelectrodes S S, are connected to electrical terminals positioned at theside surfaces of said housing wall 5 by using leads L L, formed on thesame surface as that on which said segmental electrodes are positioned.These connecting methods are illustrated as examples and should not betaken as limitative.

Referring to FIG. 7, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a suspension layer22 can be any possible electrophoretic suspension layer, such as thesuspension layer 2, 14, 18 or 21 ofFIGS. la, 2a, 3a or 4a.

The suspension layer 22 includes at least one electrophoretic materialsuspended in a suspending medium and is enclosed in a housing 3 havingtwo spaced opposed major housing walls 4 and 5, at least one of which istransparent; in the drawing the housing wall 4 is transparent.

A first transparent electrode consists of a plurality of stripelectrodes x x x which are attached to the inner surface of saidtransparent housing wall 4. A second electrode is attached to the innersurface of said housing wall 5 and consists of a plurality of stripelectrodes y y y which are parallel to each other and are orthogonal tosaid strip electrodes x x x A D.C. electric field is applied across oneelectrode selected from among strip electrodes x x x and one electrodeselected from among sn'ip electrodes y y y for example, across stripelectrodes at; and y The portion of the suspension layer 22 at theintersection of the two strip electrodes x and y is actually subjectedto a D.C. electric field and forms one picture element.

Thinner strip electrodes can be used to form smaller picture elements. Aselection of more than one strip electrode from among the stripelectrodes forming the first and second electrodes produces a desiredpattern consisting of a plurality of picture elements. Scanningtechniques known to the electrical display art can be utilized to scanthe picture elements sequentially and cyclically.

The above described electrodes having parts arranged in a given patternor being formed of a plurality of segmental or strip electrodes, asshown in FIGS. 5, 6 and 7, can be easily prepared by using any availableand suitable method such as electrodeposition, vacuum evaporation,printing or photoetching techniques.

Referring to FIG. 8, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a suspension layer22 can be any possible electrophoretic suspension layer, such as thesuspension layer 2, 14, 18 or 21 of FIGS. 1a, 2a, 3a or 4a. Thesuspension layer 22 includes at least one electrophoretic materialsuspended in a suspending medium and is enclosed in a housing 3 havingtwo spaced opposed major housing walls 31 and 32, at least one of whichis transparent; in the figure, housing wall 31 is transparent andconsists of a sheet of an insulating material such as polyester,cellulose acetate, cellophane or polyethylene.

A first electrode 30 is not adhered to the housing wall 31, but isplaced on the outer surface of the housing wall 31 so that it can beeasily removed and yet it is coupled to the suspension layer 22. Thesecond electrode is made of, for example, a metal plate and isconstituted by the other housing wall 32 as shown in FIG. 8.

When the said other housing wall has a high electrical resistance it ispossible to attach, as a second electrode, a thin electricallyconductive film to the inner surface of the other housing wall 32 or itis also possible to use, as a second electrode, a metal plate on whichsaid other housing wall 32 is placed. When the housing wall 31 has ahigh electrical resistance, a higher D. C. voltage must be appliedacross said first electrode and second electrode.

When said first electrode 30 has a given pattern, a D.C. electric fieldapplied across the first electrode 30 and second electrode 32 producesthat pattern on the surface of the housing wall 31 due to theelectrophoretic movement of the electrophoretic material, even afterremoval of said first electrode 30.

When said first electrode 30 is a pen type electrode and moves freely onthe surface of the housing wall 31, one can display any desired patternon the surface of the housing wall 31, for example writing, by applyinga D.C. voltage across said pen type electrode 30 and second electrode32, while moving said pen type electrode 30 in the said desired pattern.

. which are parallel to each other and An electric field can be appliedacross the suspension layer 22 by charging the surface of the housingwall 31 having a high electrical resistance with charged particles suchas ions or electrons in a manner similar to that well known in theelectrostatic recording art.

In order to erase the patterns produced on the display sheet by any suchmeans as described above, a conductive roller with an applied electricvoltage is caused to roll on the top surface of the insulating sheet 31or charged particles having a polarity to cause erasure are placed onthe surface of said insulating housing wall 31, so as to apply anelectric field with reversed polarity across the suspension layer 22.

It is preferable to insert between said two major housing walls 31 and32 a colorless spacer such as a porous layer 39, as shown in FIG. 8b, ora sheet 40 having a lot of projections thereon, as shown in FIG. 8c,particularly when the walls are flexible. Said colorless spacer can keepthe suspension layer 22 at a given thickness by preventing the two majorhousing walls 31 and 32 from touching each other, even when an electrodeis pressed against the flexible housing wall 31, or the flexible housing3 includes an electrophoretic suspension layer in a liquid state and isbent.

Said colorless spacer can be made from any colorless sheet having poresor projections. One appropriate material is a screen of a material suchas Tetron or nylon. The Tetron is a trade name of polyester fiber inJapan. Colored porous layer 12 in FIGS. 2a must be substantially opaqueand hide the electroporetic material, but said colorless spacer 39 or 40need not hide the electrophoretic material and can act as a spacerbetween two major housing walls.

Said colorless spacer is inserted between two major housing walls, or atleast one of the major surfaces of the colorless spacer is attached tothe inner surface ofthe housing wall.

Said colorless spacer need not be inserted in a suspension layerincluding a colored porous layer, if the colored porous layer itself canact as a spacer between two major housing walls.

Referring to FIG. 9 wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a housing 3includes an insulating housing wall 31 and a transparent housing wall 5having a transparent first electrode 9 attached thereto and connected toa source of electric power 10. The electrophoretic suspension layer 22is enclosed in the housing 3. Said housing 3 is designed to form thefront face ofa chamber 33 ofa cathode ray vacuum tube.

Negative electron charges in a given pattern are deposited on theinsulating housing wall 31 by a video modulated electron gun 34controlled by a deflecting apparatus 35 so that an electric field isapplied across the electrophoretic suspension layer 22. A visiblepattern can be reproduced on the transparent housing wall 5 due to theelectrophoretic movement of the electrophoretic material. The firstelectrode 9 acts as an anode. The second electrode is the electron gun34 which acts as a cathode. The visible pattern can be erased by thesecondary emission characteristics of the housing wall 31.

The display device shown in FIG. 9 can be modified. The insulatinghousing wall 31 can be replaced by a well-known wire-mosaic faceplateconsisting of a thin glass sheet having hundreds of fine transverselyextending wires embedded therein. This wire-mosaic provides theelectrical connection between the electron beam in the vacuum and theelec trophoretic suspension layer which is outside the vacuum. Theelectron beam charges said wire-mosaic so as to apply an input electricfield across the suspension layer 22.

Referring to FIG. 10, wherein similar reference numbers designatecomponents similar to those in the foregoing figures, a suspension layer22 can be any possible electrophoretic suspension layer, such as thesuspension layer 2, 14, 18 or 21 of FIGS. la, 20, 3a or 4a. Thesuspension layer 22 includes at least one electrophoretic materialsuspended in a suspending medium and is applied to a base plate 36, suchas paper, a metal plate or a plastic sheet. The base plate 36 is placeon a second electrode 37. Since the suspension layer 22 is not en closedin a housing, it must have high viscosity, for example it must be in asolid state at room temperature and yet it must be such that it can besoftened during application of a DC. voltage by any suitable method,such as heating or the addition of a solvent.

A first electrode 30 having a given pattern is in direct contact withthe surface of the suspension layer 22. An electric field is appliedacross the first electrode 30 and the second electrode 37, so as to movethe electrophoretic material electrophoretically while the suspensionlayer 22 is softened by heating or addition of a solvent. Removal ofsaid first electrode 30 leaves the given pattern on the of thesuspension layer 22. When the base plate 36 is transparent, one canabsorb, through the transparent base plate 36, another given patternhaving different color. A visible given pattern can be reproducedpermanently on the suspension layer 22 by cooling the suspension layeror by evaporating the solvent. When the base plate 36 is conductive, italso acts as said second electrode 37.

The aforesaid electrophoretic suspension layer can be prepared by usingany well known stable colloidal particles suspended in a suspendingmedium, such as colloidal graphit suspended in mineral oil, or colloidalsilver particles suspended in water. In addition to colloidal particles,one can use, as an electrophoretic material, finely divided particlessuch as titanium dioxide, zinc oxide, carbon black, phthalocyanine blue,phthalocyanine green, hansa yellow or watchung red suspended stably in asuspending medium such as kerosene, trichlolotrifluoroethane, isopropylalcohol or olive oil. An electrophoretic material suspended in asuspending medium usually has a positive or negative charge, dependingupon the properties of the electrophoretic material and the suspendingmedium.

The electrophoretic suspension layer 2 or 14 of FIG. 1a or FIG. 2a canconsist of only one electrophoretic material having a positive ornegative polarity suspended in a suspending medium. The electrophoreticsuspension layer 18 or 21 of FIGS. 3a or 4a must include at least twokinds of electrophoretic materials suspended in a suspending medium.Those two kinds of electrophoretic materials must have dif ferentoptical reflective properties and different charge polarities orelectrophoretic mobilities. Therefore, in preparing an electrophoreticsuspension layer 18 or 21, at least one pair of electrophoreticmaterials having suitable optical reflective properties andelectrophoretic properties must be selectively suspended in a suspendingmedium.

Average particle sizes of the finely divided particles which will beoperable are dependent upon the stability and the hiding power of theresultant electrophoretic suspension, and usually range from 0. 1,11. to50.11..

It is preferred to add any suitable and available charge control agent,dispersion agent or stabilizing agent to the electrophoretic suspensionlayer to provide a stable suspension layer in accordance with the priorart well known in solloidal chemistry. In order to control the chargeproperty of the particles suspended in a suspending medium, it ispreferred to use particles coated with resin which is not soluble in, oronly partially soluble in, the suspending medium. When the coating resinis partially soluble in the suspending medium, it can also act as afixing agent for a display image.

It is possible to use, as a suspending medium, any available andsuitable liquid which is inert to the electrophoretic material, thehousing and the electrodes. For producing a temporary display, one canuse a suspending medium in a liquid state at room temperature, i.e. from0 to 35 C. For producing a permanent display, that is, a hard copy, onecan use a suspending medium which is in a solid state at roomtemperature and in a liquid state above room temperature, i.e. above 35C. Suspending media which are operable for this purpose are, forexample, waxes such as beeswax, vegetable wax, parafiin or syntheticwax.

When using such waxes, the device according to the present inventionmust be kept at a temperature higher than room temperature for producingthe display or recording. After the device is subjected to a DC.electric filed at the higher temperature to vary electrophoretically thespatial distribution of said electrophoretic material, it is cooled toroom temperature to produce a permanent display. If it is desired toerase the permanent display, the device is subjected to an AC. or DC.electric field at the higher temperature.

When the suspending medium consists of a thermosetting material which isin a liquid state at room temperature, one can produce a permanentdisplay by heating the suspending medium after the electrophoreticmovement of the electrophoretic material.

Thermosetting materials which are operable as suspending media are, forexample, drying oil such as linseed oil, soya oil or tung oil. When theliquid suspending medium includes a binder such as polystyrol, vinylacetate resin or linseed oil which fixes the electrophoretic material ina finely divided powder form, one can obtain a hard copy having apermanently visible image reproduced thereon by evaporating orexhausting the residual suspending medium. The evaporation or exhaustingof the suspending medium can be achieved by, for example, evacuating thehousing including electrophoretic material in the suspending mediumthrough an outlet formed, for example, in the housing wall.

A housing for use in the device according to the present in vention canbe prepared by using any available material which is inert to thesuspending medium and the electrophoretic material. For example, aplastic sheet having a major center part cut out can be used for a frame38 of the housing 3 as shown in FIG. la. One of the two opposite majorhousing walls can be provided by adhering, for example, a metal plate tothe frame. The metal plate acts as one electrode of the aforesaid twoelectrodes. The other of the two opposite major housing walls can beprovided by adhering, for example, a transparent glass plate having atransparent conductive thin film such 'as tin oxide or cuprous iodine tothe frame in such a way that the transparent conductive thin film isbrought into contact with the electrophoretic suspension layer.

An electrophoretic material suspended in a suspending medium can bepoured into a housing having only one major housing wall. After that,the other major housing wall can be attached to the frame.

Another method is to pour the electrophoretic material suspended in asuspending medium into the housing through an inlet formed, for example,in the housing wall. The inlet is closed after the housing is filledwith the electrophoretic material suspended in the suspending medium.

One can use any appropriate apparatus for applying an electric fieldacross the suspension layer through the two electrodes. For example, apulse generator, a battery or any other D.C. source can be used. Inorder to erase the displayed image, an A.C. source can also be used.

It has been discovered according to this invention that when at leastone of the aforesaid first electrode and second electrode is coated withan insulating layer which is in contact with the aforesaid suspensionlayer, the device according to the present invention has an improvedoperating life. The insulating layer prevents the breakdown of theinsulating property of the suspension layer even when a high electricvoltage is applied thereto and makes it easy to remove theelectrophoretic material from the electrode surface in order to form anew image by subjecting thedevice to a D .C. electric field of reversedpolarity or to an AC. field.

Referring to FIG. 11, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, a first electrode8 is coated with an insulating layer 43 which is not soluble in thesuspending medium. Instead of the first electrode 8, a second electrode9 or both the first and second electrodes 8 and 9 can be coated withinsulating layers. The insulating layer 43 is prepared by coating thefirst electrode with, for example, vinyl acetate resin, polystyrol orgelatin. A transparent insulating layer is thus applied to a transparentelectrode attached to a transparent housing wall.

The thickness of said insulating layer 43 depends on the electricalresistance which said insulating layer and the electrophoreticsuspension layer 22 are required to have. It is preferable for operationat a low voltage that the insulating layer 43 have an electricalresistance no higher than that of the suspension layer 22.

Referring to FIG. 12a, wherein similar reference numbers designatecomponents similar to those of the foregoing figures, the suspensionlayer 22 has a plurality of spacers extending traversely thereof andconsists of many small spaces filled with the suspension. A sheet 41with a lot of holes 42 therein, as shown in FIG. 13, can be used so asto divide the suspension layer 22 into separate suspension units.

Holes 42 can have any suitable shape, such as square, as shown in FIG.13, circular, rectangular, hexagonal, and so on. Holes 42 can be regularor irregular in shape, dimension and order. The dimensions of the holes42 should be selected according to the purpose of the display or thenature of the suspension, but they must be at least greater than thedimensions of electrophoretic material in the suspension.

The advantages of dividing the suspension layer into a plurality ofsuspension units are as follows: a uniform display can be producedbecause flow of the suspension is restricted to the interior of eachspace. Said plurality of suspension units can have difi'erent opticalreflective properties from each other. It is possible for said pluralityof suspension units to have at least two colors. The plurality ofsuspension units are preferably inserted between two electrodes, one ofwhich has a plurality of segment electrodes (E E E for example, in dotform and in contact with the plurality of suspension layer units,respectively, and the other of which is transparent and covers the wholeof the transparent housing wall 4, as shown in FIG. 12b. The device asshown in FIG. 12b can produce a color image by applying a DC. voltagebetween the electrode 8 and selected segment electrodes.

The best way to provide electrodes for the plurality of suspension unitsis to provide a first electrode consisting of a plurality of stripelectrodes parallel to each other and a second electrode consisting of aplurality of strip electrodes which are orthogonal to said firstelectrodes, similar to the arrangement shown in FIG. 7, in such a waythat each of intersection points of said first electrodes and saidsecond electrodes is operatively associated with one of the plurality ofsuspension units. Each of the intersection points forms a pictureelement in association with a respective suspension unit which iscolored red, as at 22R, green as at 22G, or blue, as shown in 22B inFIG. 12c. An application of an electric field to desired pictureelements will reproduce a colored image on the image display panel.

A display panel for a color image can be prepared by using only one kindof suspension capable of changing in color in shades of grey betweenblack and white. This is accomplished by making the areas of thetransparent housing wall corresponding to each picture element in adisplay panel, as shown in FIG. 7, selectively colored so that it actsas a color filter for red, green or blue. However, a display panelhaving at least three kinds of suspensions, i.e. red, green and blue,and produce a better color image, especially with respect to lightnessof the high lights, than can a panel with mosaic color filters on thetransparent housing wall.

In a character display panel, as described in FIG. 6, the suspensioncorresponding to one of the segmental electrodes can consist of one ormore suspension units.

The amount of electrophoretic material in a suspending medium or thethickness of the electrophoretic suspension layer is selected, dependingupon the hiding power or electrophoretic property of the electrophoreticmaterial, the range of color change required in the device, thefeasibility of the voltage source and so on. The value of the hidingpower of pigment particles available commercially is helpful inpreparing the suspension. For example, the values of the hiding powersof titanium dioxide particles and acetylene black particles are about cmand 25,400 cm respectively, per gm in the conventional suspendingliquids such as linseed oil. Therefore, at least 1 gm of titaniumdioxide particles and at least 5 mg acetylene black particles arerequired, respectively, so as to change the color of the display panelhaving an area of 130 cm from sufficient black to sufiicient white. Whenthe thickness of suspension layer is to be 1 mm, said amount of eachpigment particles must be introduced into 13 ml of the suspendingmedium.

Since the display device of the present invention is the reflectivetype, the suspension layer must be opaque in order to make a great colorchange. The thicker the suspension layer, the higher the applied voltagewhich is usually required. The thinner the suspension layer, the denserthe concentration of the electrophoretic material must be to make agreat color change. The thickness of the suspension layer is usuallyfrom a few microns to a few mm.

EXAMPLE 1 A mixture of particles as described in Table l is combinedwith 100 ml of iso-propyl alcohol. The mixture in the isopropyl alcoholis well blended to produce a grey paste having white and black particlessuspended in iso-propyl alcohol. In this grey paste, the titaniumdioxide particles are positively charged and the black toner particlesare negatively charged. A housing as shown in FIG. 1 is filled with thegrey paste so as to form an electrophoretic suspension layer. Thehousing has two opposite major housing walls with a size of 60 X 60 mm.The two major housing walls are made of transparent electrodes, each ofwhich is a thin film of tin oxide applied to a transparent glass plate.This electrically conductive glass is called EC glass in the followingexamples. A side frame is prepared from 25,11. thick polyester film in amanner similar to that described above. The thickness of theelectrophoretic suspension layer is designed to be 25 1.. Thiselectrophoretic suspension layer is almost opaque and grey when viewedthrough either of said transparent electrodes under the light of anincandescent lamp.

TABLE 1 Titanium dioxide:

g (a brand R-68O commercially available from the lshihara IndustrialCompany in Japan, of rutile type having a particle size 0.15 0.3 1..

black toner particles:

g (Type-1O manufactured for electrophotography by the Rank Xerox Companyin England).

When a DC. voltage of V is applied between the two electrodes, thesuspension layer is changed in color; it is black at the anode and whiteat the cathode. The black color at the anode or the white color at thecathode remains stable even after the DC. voltage is cut off.

The color on one side can be changed from black or white to white orblack by reversing the polarities of the applied voltage. An applicationof an A.C. voltage of 25V at 60 Hz between the two electrodes changesthe color from the white at the cathode or the black at the anode togrey at both electrodes, and it remains in this original condition.

When the applied AC. voltage has a frequency below about 20Hz, thelightness of the suspension layer is altered periodically from the whitethrough grey to black and vice versa in accordance with the frequency ofthe applied voltage. An application of DC. voltage below 25V changes thecolor gradually and continuously from grey to white at the cathode orblack at the anode. A desired grey color between the white and the blackwill remain in a stable condition after switching off the voltage at themoment when the desired color appears on the panel.

A rapid change in the lightness of the suspension layer can be obtainedby applying a DC. voltage above 25V with a definite pulse width. Thecolor ofthe panel is simply and easily changed and the panel is usefulfor conveying information.

EXAMPLE 2 Fifteen grams of finely divided titanium dioxide particles(same as in Table l) and fifteen grams of black toner particles (same asin Table l) are added to 200 ml ofolive oil.

The mixture is well blended in a ball mill to produce a grey paste. Inthis grey paste, the titanium dioxide particles are positively chargedand the black toner particles are negatively charged. A housing similarto that of Example 1 is filled with the grey paste so as to produce acell having an electrophoretic suspension layer as shown in FIG. 3. Thethickness of the electrophoretic suspension layer is g.

The side faces of said cell are made liquid-tight by an adhesive agent,for example, Araldite, an adhesive commercially available from CIBALimited in Switzerland, while each electrode is partially exposed toallow for connection of leads.

The application ofa DC. voltage of 250V between the electrodes for aboutl-second changes the color of the suspension layer at each electrode;one observes a change in color from grey to white at the cathode and toblack at the anode. A current meter with a recording apparatus connectedbetween the voltage source and the cell indicates that the averagecurrent flowing during electrophoresis of particles is about 4 X lOamperes per cm of the cell. This means that the display apparatus of thepresent invention has a rather high electrical efficiency. By theapplication of a DC. voltage below 250V, the lightness of the suspensionlayer can be altered slowly and it remains stable at the desiredlightness level.

EXAMPLE 3 Ten grams of Heliogen blue LBGT particles, which arephthalocyanine blue supplied by the BASF Company in Germany, is added to100 ml of olive oil and blended well in a ball mill so as to produce ablue paste. Fifteen grams of hansa yellow G particles, which are an azotype organic pigment supplied by the Kanto Chemcial Company in Japan, isadded to 100 ml of olive oil and blended well in a ball mill so as toproduce a yellow paste. Equal amounts of the two pastes are mixed wellto produce a green paste.

The green paste is placed between an aluminum plate and an EC glasselectrode so as to produce an electrophoretic suspension layer with athickness of 25,14. When a DC. voltage of 100V is applied between the ECglass electrode as an anode and the aluminum plate, the color of thesuspension layer as viewed through the glass electrode changes fromgreen to yellow under a white light. The color of the suspension layeris changed to blue by reversing the polarity of the applied voltage.

Control of strength, the length of time of application or the polarityof the applied voltage makes it possible to change the color of saidsuspension layer continuously from the color of the yellow particlesthrough that of a mixture of yellow particles and the blue particles tothat of the blue particles and vice versa. This is because thesuspension layer adjacent to the EC. glass electrode includes a mixtureof blue and yellow particles in various ratios.

Said cell is thus useful as a color changeable panel capable of alteringthe color of the suspension layer continuously throughout the colorrange of the colors blue, green, and yellow.

EXAMPLE 4 Four grams of hansa yellow G particles (same as in Example 3)is added to 50 ml of olive oil and blended well in a ball mill toproduce a yellow paste.

Eight grams of ultramarine particles supplied by the Daiichi KaseiIndustrial Company in Japan is added to 50 ml of olive oil and blendedwell in a ball mill to produce a blue paste.

Equal amounts of the pastes are mixed well to produce a green paste. Ahousing which employs, as two opposite major housing walls, two flexibletransparent electrodes made from a sheet of cellulose diacetate (CDA)and having a transparent conductive layer of cuprous iodine (Cul)applied to each wall is filled with said green paste. A sheet of Tetronscreen (01350 a screen woven of polyester fibers supplied by the TeijinCompany in Japan) is inserted between said two flexible transparentelectrodes in the manner shown in FIG. 8b.

When a DC. voltage of 200V is applied between the two flexibletransparent electrodes, the anode becomes yellow and the cathode becomesdark blue-green. A reversal of the polarity of the applied voltagecauses the colors of the two electrodes to change from the yellow to theblue-green and from the glue-green to the yellow, respectively.

Hansa yellow G particles are negatively charged, but ultramarineparticles do not have any great electrophoresis activity in olive oil.In this suspension layer, the yellow particles capable of movingelectrophoretically are suspended in bluecolored suspending mediumconsisting of olive oil colored by ultramarine particles.

Said cell is useful as a flexible sheet capable of changing colorthroughout a color range from yellow through green and blue-green,depending on the change in strength, length of time of application, andpolarity of the applied voltage.

EXAMPLE A mixture of particles described in Table 2 is combined with 50ml of iso-propyl alcohol. This mixture is iso-propyl alcohol is wellmixed by an ultrasonic vibrator to produce a yellowgreen suspension.

TABLE 2 Cadmium sulfide particles:

g (commercially available from the Sakai Chemical Industry Company inJapan) Patent blue A particles:

2g (commercially available from the Kanto Chemical Industry Company inJapan).

A housing is filled with the suspension so as to form an electrophoreticsuspension layer, as shown in FIG. 1. Two major housing walls of thehousing are made of EC glass electrodes. The side frame is made of apolyester film. The thickness of the electrophoretic suspension layer is25,.

When a DC voltage of V is applied between the electrodes, the suspensionlayer is changed in color. It is yellow at the cathode and green at theanode. The color on the respective sides can be changed from yellow orgreen to green or yellow by reversing the polarity of the appliedvoltage.

EXAMPLE 6 Thirty-five grams of a cobalt naphthenate, metal content 8weight percent (supplied by the Kanto Chemical Company in Japan), isdissolved in 100 ml of trichlorotrifiuoroethane to produce ared-brownish solution. Twenty grams of titanium dioxide particles (sameas in Table 1) is added to the solution and blended well to produce afaint pink paste. In this paste, the titanium dioxide particles arepositively charged. A housing is filled with the paste so as to form anelectrophoretic suspension layer.

Two major housing walls of the housing are made of an EC glasselectrode. The thickness of the suspension layer is 50,u.. When a DC.voltage of SUV is applied between the two electrodes, the suspensionlayer is changed in color. It is white at the cathode and brown at theanode.

Control of the strength, the length of time application and polarity ofthe applied voltage changes the lightness of brown color at one side ofthe panel continuously. The established color remains stable even afterremoval of the applied voltage.

In the suspension as described above, the cobalt naphthenate functionsas the coloring agent for the transparent suspending medium consistingof trichlorotrifluoroethane, the charge control agent for advancing thepositive charge of the titanium dioxide particles, and the dispersingagent for stabilizing the suspension.

EXAMPLE 7 Ten grams of titanium dioxide particles (same as in Table 1 isadded to ml of olive oil and blended well in a ball mill so as toproduce a white paste.

A housing is provided which is similar in construction to that of FIG.2a. The two opposite major housing walls are formed of an aluminum plateand an EC glass electrode which act as first electrodes and secondelectrodes, respectively. The EC glass electrode has an insulating layerof vinylacetate resin having a thickness of about 7p. applied to the SnOcoating. A thick dark blue cloth is inserted as a colored porous layerbetween said first and second electrode. Said thick dark blue cloth hasa thickness of about 100,14. One thick dark blue cloth which can be usedis Bemberg which is the trade name of cloth manufactured at the AsahiKasei in Japan. A sheet of Tetron screen No. 1000 (supplied by theTeijin Company in Japan) is placed between the aluminum plate and thickdark blue cloth.

A sheet of another Tetron Screen No. 1350 (supplied by the TeijinCompany) is placed between the insulating layer on the EC glasselectrode and the thick dark blue cloth. The housing is filled with theabove described white paste. The thick dark blue cloth and Tetronscreens have the white paste impregnated therein. The panel has a faintblue color at the EC glass electrode before application of a DC.voltage.

When a DC. voltage of 400V is applied between the EC glass electrode asa cathode and the aluminum plate as an anode, the panel has a whitecolor at the EC glass electrode. This is because white titanium dioxideparticles in the paste move electrophoretically to the EC glasselectrode and hide the thick dark blue cloth from sight at the EC glasselectrode.

The color of the panel at the EC glass electrode changes from white toblue upon reversal of the polarity of the applied voltage.

EXAMPLE 8 An SnO- layer on an EC glass electrode is coated with aphotoresist solution, for example KPR, available commercially from theKodak Company in U.S.A., and is etched by a well known photoetchingtechnique so as to form segmental electrodes as shown in FIG. 60. Forconvenience, the EC glass electrode having these segmental electrodesformed thereon is hereinafter called a transparent electrode. A housingis prepared by using this transparent electrode as one electrode and analuminum plate as another electrode in a way similar to that describedin the foregoing description. The inner surface of the aluminumelectrode is coated with an insulating layer of vinyl acetate resinhaving a thickness of about 8,14.

The housing is filled with a grey paste exactly the same as that ofExample 2. The grey paste forms a suspension layer having a thickness of25a. The segmental electrodes are connected, through switches, to oneterminal of a DC. voltage source. The EC glass electrode has a greycolor before a DC. voltage is applied.

The other terminal of the DC. voltage source is connected to thealuminum electrode. An application of a DC. voltage of 200V between thealuminum electrode and selected segmental electrodes produces a desirednumerical character on the EC glass electrode.

For example, selection of segmental electrodes S S S S S 8-, (referencesas in FIG. 6) produces 0," the selection of segmental electrodes S 43produces 8; the selection of segmental electrodes S; and S produces 1.When the segmental electrodes are charged positively, the numericalcharacter which is produced is black; the opposite charge produces awhite numerical character.

What is claimed is:

l. A device for display and/or recording, comprising an electrophoreticsuspension layer, said suspension layer being selected from the groupconsisting of a suspension including at least one electrophoreticmaterial suspended in a colored suspending medium, a suspensionincluding at least one electrophoretic material suspended in asuspending medium having further a porous layer therein, a suspensionincluding at least two electrophoretic materials suspended in asuspending medium, one of said at least two electrophoretic materialsdiffering in charge polarity and optical reflective property from theother, and a suspension including at least two electrophoretic materialssuspended in a suspending medium, one of said at least twoelectrophoretic materials differing in electrophoretic mobility andoptical reflective property from the other, said suspension layer havingtwo opposite major surfaces; a first electrode and a second electrodewhich are spaced from and opposed to each other and are positioned onsaid two opposite major surfaces of said suspension layer, respectively;and means coupled to said electrodes for applying an electric fieldacross said suspension layer through said electrodes, said electricfield changing the spatial distribution of said electrophoreticmaterial, the optical reflective property of said suspension layer beingchangeable in response to said change in spatial distribution.

2. A device as claimed in claim 1 wherein said suspension layer includesat least two electrophoretic materials, one of which differs in thecharge polarity and optical reflective property from the other.

3. A device as claimed in claim 1 wherein said suspension layer includesat least two electrophoretic materials, one of which differs inelectrophoretic mobility and the optical reflective property from theother.

4. A device as claimed in claim 1, wherein said suspending medium iscolored.

5. A device as claimed in claim 1 further comprising a porous layerinserted in said suspending medium.

6. A device as claimed in claim 1 wherein at least one ofsaid firstelectrode and second electrode is in the shape of a given pattern.

7. A device as claimed in claim 1 wherein said suspension furthercontains a binder for said electrophoretic material, which binder is atleast partially soluble in said suspending medium.

8. A device as claimed in claim 1, wherein said suspending medium is ahardenable material.

9. A device as claimed in claim 1 wherein said suspending medium is heathardenable.

10. A device as claimed in claim 1 wherein said suspending medium isheat softenable.

11. A device as claimed in claim 1 wherein said suspending medium issolvent softenable.

12. A device as claimed in claim 1 wherein said means for applying anelectric field includes means for controlling said electric field withrespect to at least one property selected from the group consisting ofstrength, length of time of application and polarity.

13. A device as claimed in claim 1 further including a housing enclosingsaid suspension layer.

14. A device as claimed in claim 13 wherein at least one of said firstelectrode and second electrode is in the shape of a given pattern.

15. A device as claimed in claim 14 further comprising a conductive leadextending out of said housing.

16. A device as claimed in claim 13, wherein said suspension layer has aplurality of spacers therein extending transversely thereof.

17. A device as claimed in claim 13, wherein at least one of said firstelectrodes and second electrodes is coated with an insulating layer whisis in contact with said suspension layer.

18. A device as claimed in claim 13 in which said housing has two spacedopposed major housing walls between which said suspension layer and saidelectrodes are positioned, and at least one of which walls istransparent.

19. A device as claimed in claim 13 wherein at least one of said firstelectrode and second electrode is transparent and the housing has atransparent housing wall covering said transparent electrode.

20. A device as claimed in claim 19 wherein said first electrode coversthe whole of one surface of said suspension layer, and said secondelectrode covers the whole of the other surface of said suspensionlayer.

21. A device as claimed in claim 19 wherein at least one of said firstelectrode and second electrode is in the shape of a given pattern.

22. A device as claimed in claim 19 wherein at least one of said firstelectrode and second electrode is divided into a plu rality of segmentalelectrodes.

23. A device as claimed in claim 19 wherein said first electrodeconsists of a plurality of strips of electrode material which areparallel to each other, and said second electrode consists of aplurality of strips of electrode material which are parallel to eachother and are orthogonal to said first electrode strips.

24. A device as claimed in claim 13 wherein said housing has twoopposite major walls, and one ofsaid electrodes forms one ofsaid twoopposite major housing walls.

25. A device for display, comprising an electrophoretic suspension layerhaving a suspending medium and at least one electrophoretic material ina finely divided powder form suspended in said suspending medium, saidsuspension layer having two opposite major surfaces, a transparentelectrode positioned on one of said major surfaces of said suspensionlayer, a housing having said suspension layer enclosed therein andhaving a transparent housing wall over said electrode, and a cathode rayvacuum tube having an electron gun therein and having said housingclosing the end of said vacuum tube opposite from said electron gun withthe transparent housing wall on the outside of the vacuum tube and withsaid other major surface of the suspension layer facing toward said electron gun.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,668,106 Dated June 6, 1972 Inventor(s) ISAO OTA It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the printed heading of the patent after line 5 insert:

-FOREIGN APPLICATION PRIORITY DATA April 23, 1969 Japan. 44/31598 April23 1969 Japan. 44/31602 July 29 1969 Japan. 44/60154 June 12, 1969Japan.. 44/46698 June 12, 1969 Japan.... 44 46699 June 12, 1969 Japan..44/46700 June 12 1969 Japan. 44/46701 Column 5 lines 64-67 to bedeleted.

Signed and sealed this 10th day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCI-IER,JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents 2% UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 316681106 Dated June 6, 1972 Inventor(s) ISAO O'I'A It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the printed heading of the patent after line 5 insert:

-FOREIGN APPLICATION PRIORITY DATA April 23, 1969 Japan........ 44/31598April 23 1969 Japan. 44/31602 July 29, 1969 Ja an..-...... 44/60154 June12, 1969 Japan.. 44/46698 June 12, 1969 Japan.... 44/ 16699 June 12 1969Japan. 44/ 16709 June 12, 1969 Japan........ 44/46701 Column 5, lines64--67 to be deleted.

Signed and sealed this 10th day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. A device as claimed in claim 1 wherein said suspension layer includesat least two electrophoretic materials, one of which differs in thecharge polarity and optical reflective property from the other.
 3. Adevice as claimed in claim 1 wherein said suspension layer includes atleast two electrophoretic materials, one of which differs inelectrophoretic mobility and the optical reflective property from theother.
 4. A device as claimed in claim 1, wherein said suspending mediumis colored.
 5. A device as claimed in claim 1 further comprising aporous layer inserted in said suspending medium.
 6. A device as claimedin claim 1 wherein at least one of said first electrode and secondelectrode is in the shape of a given pattern.
 7. A device as claimed inclaim 1 wherein said suspension further contains a binder for saidelectrophoretic material, which binder is at least partially soluble insaid suspending medium.
 8. A device as claimed in claim 1, wherein saidsuspending medium is a hardenable material.
 9. A device as claimed inclaim 1 wherein said suspending medium is heat hardenable.
 10. A deviceas claimed in claim 1 wherein said suspending medium is heat softenable.11. A device as claimed in claim 1 wherein said suspending medium issolvent softenable.
 12. A device as claimed in claim 1 wherein saidmeans for applying an electric field includes means for controlling saidelectric field with respect to at least one property selected from thegroup consisting of strength, length of time of application andpolarity.
 13. A device as claimed in claim 1 further including a housingenclosing said suspension layer.
 14. A device as claimed in claim 13wherein at least one of said first electrode and second electrode is inthe shape of a given pattern.
 15. A device as claimed in claim 14further comprising a conductive lead extending out of said housing. 16.A device as claimed in claim 13, wherein said suspension layer has aplurality of spacers therein extending transversely thereof.
 17. Adevice as claimed in claim 13, wherein at least one of said firstelectrodes and second electrodes is coated with an insulating layer whisis in contact with said suspension layer.
 18. A device as claimed inclaim 13 in which said housing has two spaced opposed major housingwalls between which said suspension layer and said electrodes arepositioned, and at least one of which walls is transparent.
 19. A deviceas claimed in claim 13 wherein at least one of said first electrode andsecond electrode is transparent and the housing has a transparenthousing wall covering said transparent electrode.
 20. A device asclaimed in claim 19 wherein said first electrode covers the whole of onesurface of said suspension layer, and said second electrode covers thewhole of the other surface of said suspension layer.
 21. A device asclaimed in claim 19 wherein at least one of said first electrode andsecond electrode is in the shape of a given pattern.
 22. A device asclaimed in claim 19 wherein at least one of said first electrode andsecond electrode is divided into a plurality of segmental electrodes.23. A device as claimed in claim 19 wherein said first electrodeconsists of a plurality of strips of electrode material which areparallel to each other, and said second electrode consists of aplurality of strips of electrode material which are parallel to eachother and are orthogonal to said first electrode strips.
 24. A device asclaimed in claim 13 wherein said housing has two opposite major walls,and one of said electrodes forms one of said two opposite major housingwalls.
 25. A device for display, comprising an electrophoreticsuspension layer having a suspending medium and at least oneelectrophoretic material in a finely divided powder form suspended insaid suspending medium, said suspension layer having two opposite majorsurfaces, a transparent electrode positioned on one of said majorsurfaces of said suspension layer, a housing having said suspensionlayer enclosed therein and having a transparent housing wall over saidelectrode, and a cathode ray vacuum tube having an electron gun thereinand having said housing closing the end of said vacuum tube oppositefrom said electron gun with the transparent housing wall on the outsideof the vacuum tube and with said other major surface of the suspensionlayer facing toward said electron gun.